In order to adequately view an object by a magnifying device such as a microscope, it is very important to sufficiently illuminate the viewed object. Generally, as the magnification is increased, more light must be projected onto the object. A lack of sufficient illumination will make the object at least difficult, if not impossible, to observe. However, applying light to the object must be done in a manner which does not substantially affect the observation of the object. That is, the process of adding additional light may actually interfere with or obscure the user's ability to clearly observe the object.
One of the most common means to light an object is to back-light it. As the name implies, back-lighting simply involves placing a light source behind the object such that the light passes from its source through the object and then into the microscope. However, when viewing substantially or completely opaque objects, the light must be applied to the surface(s) being observed. Typically, opaque objects are simply lit from a source positioned at the side of and somewhat back from the object. This method of lighting is normally referred to as "side illumination".
An inherent problem with side illumination is that as the degree of magnification increases, the lighting becomes less and less sufficient. This results from the fact that as the magnification increases the distance from the objective lens to the object decreases. This distance is known as the `working distance`. High power microscopes generally have working distances on the order of a tenth of a millimeter or smaller. At such small distances, little light can be projected from the side of the microscope onto the surface of the object. Further, the light which does reach the object will be directed very nearly parallel to the plane of the object's surface, causing unwanted shadows and distortions. This is especially problematic when the surface is not (relatively) smooth.
An alternative to side illumination is vertical illumination. With vertical illumination light is transmitted from behind and through the microscope's objective lens onto the object. This requires that light be projected from within the microscope itself. After reaching the surface of the viewed object, the light reflects off the surface and some of it travels back up through the objective and eyepiece lenses to viewer. The amount of light returning to the microscope depends on the reflectiveness of the object's surface.
If light is projected from within the microscope itself steps must be taken to avoid interference with viewing the object. Perhaps the most common type of vertical illuminator attempts to avoid interference by positioning its light source off to a side of the microscope tube and using a beam-splitter to direct the projected light down through the objective lens and onto the object. That is, the light source is positioned off to the side of the tube and transmits its light roughly perpendicular to the axis of the microscope, through an opening in the side of the tube between the objective and eyepiece lenses. A beam-splitter is placed about the microscope's optical axis at generally a 45 degree angle so to direct the light towards the objective lens, generally parallel with the microscope's axis. The beam-splitter is typically a partially silvered piece of glass which allows about half the light to pass directly through and reflects the remaining light. When the light is traveling back up through the microscope, from the objective lens towards the viewer, the beam-splitter again allows only amount half the light to pass through to the viewer and reflects back towards the light source the remaining light.
Therefore, at best with a beam-splitter, the microscope only allows about a quarter of the light emitted by the lamp to reach the viewer. The stray light which is diverted by the beam-splitter must be controlled to prevent interference with the viewer's image. To overcome these reductions in light, an intense light source may be required. Of course, increasing the size of the light source increases the cost and the heat produced. Another problem with using a beam-splitter to direct the projected light is that any defects or aberrations present in the beam-splitter will affect the quality of the viewer's image. Of course, the defects and aberrations can be minimized by precision fabrication, which also increases costs. Thus, the use of beam-splitters to direct light presents some significant problems.
In other vertical illuminator designs, mirrors and/or prisms are used in place of beam-splitters to direct the light projected from the light source. Although these elements tend to project more of the light they direct, they (and their associated supporting structures) act to block some of the light returning from the objective to the viewer. These elements must be positioned with the defocused region existing behind the objective lens to avoid inference with the image of the object. Even so positioned, the blocking of light will reduce the brightness of the object image. The larger the mirror or prism, the greater the decrease in illumination will be. Thus, the use of a mirror or a prism does little to overcome the problems associated with the prior devices.
Further problems exist as beam-splitters, mirrors and prisms are all inherently difficult to adjust to obtain proper directing of the projected light. Adjustment may be critical to the operation of the microscope not only to adequately light the object, but also to avoid interference caused by projected light reflecting off the back surface of the objective lens and returning to the viewer. Some prior devices employing mirrors to direct the projected light have had to resort to tilting the objective lens to avoid reflections. Obviously, such a complicated modification is undesired as it prevents the use of the illuminator in commercial microscopes with interchangeable objective lenses of varying powers.
A further problem is that the addition of beam-splitters, mirrors, and/or prisms and their supporting structures, increases cost, size, weight and complexity. They make the microscopes more fragile and susceptible to misalignments. Also, the inclusion of the additional components can present problems when the microscope is used in unique applications such as within a vacuum, where all components must be specifically prepared to prevent out-gassing from certain materials or trapped air.
Thus, a device is sought which will direct light in a manner that provides sufficient illumination of the viewed object without substantially blocking, reducing and/or distorting the light traveling back up through the microscope to the viewer. In so doing the device should use the smallest necessary light source to reduce cost and complexity. The device must be easy to adjust to prevent unwanted reflections causing light interference at the eyepiece. The device must also be compatible and easily integrated into existing commercial microscopes having interchangeable objective lenses of varying degrees of magnification. Further, the device must be capable of operating in specialized environments such as vacuums with minimal out-gassing. The device must also be sturdy, durable and relatively low in complexity, cost and weight.